Merge #11121: TestNode tidyups

7148b74dc [tests] Functional tests must explicitly set num_nodes (John Newbery)
5448a1471 [tests] don't override __init__() in individual tests (John Newbery)
6cf094a02 [tests] Avoid passing around member variables in test_framework (John Newbery)
36b626867 [tests] TestNode: separate add_node from start_node (John Newbery)
be2a2ab6a [tests] fix - use rpc_timeout as rpc timeout (John Newbery)

Pull request description:

  Some additional tidyups after the introduction of TestNode:

  - commit 1 makes TestNode use the correct rpc timeout. This should have been included in #11077
  - commit 2 separates `add_node()` from `start_node()` as originally discussed here: https://github.com/bitcoin/bitcoin/pull/10556#discussion_r121161453 with @kallewoof . The test writer no longer needs to assign to `self.nodes` when starting/stopping nodes.
  - commit 3 adds a `set_test_params()` method, so individual tests don't need to override `__init__()` and call `super().__init__()`

Tree-SHA512: 0adb030623b96675b5c29e2890ce99ccd837ed05f721d0c91b35378c5ac01b6658174aac12f1f77402e1d38b61f39b3c43b4df85c96952565dde1cda05b0db84